1. Field of the Invention.
The present invention relates to the production of liquid nitrogen as a by-product in a natural gas liquid (NGL) recovery plant, and more particularly, to a method of liquefying nitrogen separated from a natural gas feed stream whereby a portion of the methane-rich residue gas produced is utilized as the refrigerant for condensing the nitrogen.
2. Description of the Prior Art.
Various methods have heretofore been developed and utilized for recovering natural gas liquids from natural gas streams comprised predominantly of methane, significant amounts of ethane and heavier hydrocarbons, and significant amounts of nitrogen. Such methods, as utilized in NGL recovery plants, generally produce a separated methane-rich residue gas stream which is sold or utilized as heating fuel, and an ethane and heavier liquid hydrocarbon product stream. The liquid hydrocarbon stream is usually fractionated to produce propane and butane liquefied petroleum gas (LPG) and pentane and heavier hydrocarbons which are valuable as blending stocks for motor fuels and for other purposes. In addition to the useful components, natural gas feed streams generally also contain undesirable amounts of acid gases, i.e., carbon dioxide and hydrogen sulfide, as well as water, all of which must be removed so that they do not reduce the heating value of the methane-rich residue gas stream or cause other problems. Nitrogen, often present in natural gas feed streams in relatively high amounts, has also been considered an undesirable impurity in the product heating gas stream, and has been removed therefrom in the form of a nitrogen-rich gas stream which has either been vented to the atmosphere or conducted to a point of use such as an enhanced oil recovery project where nitrogen is injected into the oil-containing reservoir.
An example of a prior art method of recovering natural gas liquids while simultaneously rejecting nitrogen from the product residue gas stream is disclosed in U.S. Pat. No. 4,746,342 issued May 24, 1988. In that method, a natural gas feed comprised predominantly of methane and containing significant amounts of ethane and heavier hydrocarbons and nitrogen is separated into a vapor phase and a liquid phase. The separated liquid phase is fractionated to recover ethane and heavier hydrocarbons as liquid products and a pipeline gas as a vapor phase product. The vapor phase from the separation step is sequentially fractionated in second, third and fourth fractionation steps to produce additional product pipeline gas, an in-plant fuel and a substantially pure gaseous nitrogen stream which is vented to the atmosphere or conducted to a point of use.
U.S. Pat. No. 4,411,677 issued Oct. 25, 1983 discloses a process for rejecting nitrogen from a natural gas feed under elevated pressure using a single distillation column and a closed loop methane heat pump which reboils and refluxes the column. The rejected nitrogen is yielded as a gas.
Another example is the method disclosed in U.S. Pat. No. 3,205,669 issued Sept. 14, 1965 wherein helium concentrate, a residue gas rich in methane, natural gas liquids and a substantially pure nitrogen gas stream are produced.
By the present invention, an improved method of separating a natural gas feed stream into at least a hydrocarbon liquid product stream, a methane-rich gas product stream and a substantially pure liquefied nitrogen product stream is provided.